The various embodiments of the invention relate to a blade for a wind power plant and a wind power plant comprising a tower and a rotor with a number of blades, of which at least one blade is mounted to be pitch-adjustable in a pitch bearing. Moreover, the various embodiments of the invention relate to methods of controlling such wind power plant.
The achievable electricity output of a wind power plant depends directly on the size of the rotor area and hence on the effective length of the blades. A blade which is 1% shorter will thus result, as a rule of thumb, in a 2-3% lower electricity output. As a result of efforts to save material and weight, the blades of a wind power plant are often very flexible, and the flexing due to the wind may thus be quite comprehensive. It follows that the deformation of the blades lead to a reduction in the rotor area and hence to an undesirable reduction in the power yield.
Moreover, the deformation of the blades is often a limiting dimensioning factor to the design of new wind power plants, as one has to make sure that the blades do not hit the tower. Positioning of the rotor a further distance from the tower is not desirable, as an increased length of the main shaft giving rise to a larger momentum on the tower and hence to undesirable forces in is gears and bearings in the hub.
Depending on the velocity of the wind, it may thus be desirable to both increase the rotor area to better utilize the wind and to increase the power output (at low wind velocities) and to change the shape of the blades in order to avoid them hitting the tower (at high wind velocities).
It is known from EP 1019631 to manufacture pre-curved blades that partially compensate for the flexing caused by the wind. However, the complete useful length of the blade is still achieved only at precisely the specific design wind velocity. At all other wind velocities the blade will still either flex up into the wind or rearwards.
Other methods of ensuring that the blades stay clear of the tower is by coning where the blades are mounted forwards into the wind, forming a cone, or by tilting where the main shaft as such is, with its entire rotor plane, turned upwards in the order of about 5°. However, both of these construction methods entail a reduction in the effective rotor area compared to blades of a specific length and hence a reduction in the achievable power yield.
A wind power plant regulates the power uptake at different wind velocities by means of the blades in accordance with three methods.
Like a plane may lose lift and stall, a blade can be turned to lose lift and the output of the rotor can be reduced. On passively stall-adjusted turbines, each blade is fixedly mounted on the hub in a specific angle of attack. The blade is constructed such that turbulence is generated on the rear side when it is very windy. Such stall discontinues the lift of the blade. The more powerful the wind, the heavier the turbulence and the ensuing braking effect, whereby the power output of the blade is regulated.
Actively stall-adjusted wind turbines turn the rear edge of the blade a few degrees up into the wind (negative pitch angle) when they regulate the output. It takes place by the entire blade being turned (pitched) about its own axis—most often by means of a hydraulic system.
The majority of rotors on recent and large wind power plants are pitch-adjusted. Here the power output is regulated in accordance with the wind conditions in that the leading edge of the blade is turned up into the wind (positive pitch angle) as opposed to the above actively stall-adjusted turbines that turn the rear end of the blade up into the wind.